cxgb4: Adds support for T6 adapter

Adds NIC driver related changes for T6 adapter. Register related
changes, MC related changes, VF related changes, doorbell related
changes, debugfs changes, etc

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>